Leakage reducing means



May 12, 1942. w. J. BROWN LEAKAGE REDUCING MEANS Filed Feb. 28, 1941 3 Sheets-Sheet l ATTOR NEY May 12, 1942. w. J. BROWN LEAKAGE REDUCING MEANS Filed Feb. 28, 1941 3 Sheets-Sheet 2 ilk/22 gl onm' ATTORNEY y 1942- w. J. BROWN 2,282,319

LEAKAGE REDUCING MEANS Filed Feb. 28, 1941 5 Sheets-Sheet 3 Patented May 12, 1942 LEAKAGE REDUCING MEANS Walter J. Brown, Cleveland Heights, Ohio, as-

signor to The Brush Development Company, Cleveland, Ohio, a corporation of Ohio Application February 28, 1941, Serial No. 381,159

24 Claims.

This invention relates to a method and apparatus for improving the performance of high impedance alternating current signal generators, and it relates more particularly to a method for mitigating the effects of signal current leakage in generators of the type described.

High impedance signal generators, of which the piezo-electric unit is an example, find many commercial applications, but are restricted in their usefulness by the fact that they are essentially voltage generators rather than current generators. This will be understood when it is recognized that if current is drawn from the generator, much of its generated voltage will be developed across its high internal impedance and little will appear at the generator terminals. On the contrary, if no current is drawn from the generator, all of its generated voltage will appear at the terminals, and will be available for use. This limitation in high impedance generators has long been recognized, and is kept in mind in designing the load circuits to which such generators are connected. Thus, if the generator is to be connected to a vacuum tube amplifier, it is customary to design the amplifier to have a high impedance input circuit. If the latter impedance is at least as high as the internal impedance of the generator, adequate signal voltage will ordinarily be obtained in the input circuit to control the amplifier. As the input impedance is increased in relation to the impedance of the generator, more voltage becomes available for use in controlling the amplifier. If the input impedance is made very large compared with the generator impedance so that very little or no current is drawn from the generator, the maximum amount of control voltage is made available. Amplifiers having extremely high input impedances, approaching infinite values, have become a reality through the invention disclosed in the copending application of J. P. Arndt, Jr., Serial Number 370,773, filed December 19, 1940, so that it now is possible to connect high impedance generators to amplifiers which draw little or no current from the generator. Heretofore, however, the advantages of such combinations could not be realized to their full extent if the generator had stray leakage paths of finite impedance. Such leakage paths are, of course, in shunt with the generator and with the impedance of the input circuit of the amplifier. Consequently, such leakage paths act as load circuits which draw current from the generator, and thereby defeat the objects sought to be accomplished through the use of the high impedance load, or input circuit. However, through the invention described hereafter in detail, it is possible to treat the leakage paths in a manner which mitigates, and in many instances avoids, the detrimental effects of such leakage currents.

The advantages of the present invention are particularly useful in connection with electrostatic generators such as condenser and electric microphones, and piezoelectric units and de vices. Such electrostatic devices are inherently of a capacitive nature, whereas the leakage paths are essentially resistive. The result is that the leakage currents introduce an undesired low frequency cutoff. That is, for alternating current signals within a wide range of frequencies, the internal impedance of the generator may be sufficiently low compared with the resistance of the leakage path that the leakage currents do not materially reduce the voltage which is available at the generator terminals. At frequencies below this range, however, the internal impedance becomes so high that the leakage currents cause most of the generated signal voltage to be developed across the internal impedance, and very little to appear at the terminals. In consequence thereof, the generator becomes substantially ineffective as a generator of these low frequency signals. By means of this invention, however, the effective impedance of the leakage paths can be raised high enough to avoid their effects, with the result that the generator may be worked at much lower frequencies.

The effectiveness of the present invention in connection with electrostatic generators is illustrated by the sound cell microphone such as that disclosed in Sawyer Patent No. 2,105,010 and Williams Patent No. 2,126,438. Inasmuch as the output of each piezoelectric unit in the sound cell is relatively small, it is usually necessary to connect a plurality of the units in series. The series connection results, however, in a small capacity for the microphone and thus requires a high input impedance in the amplifier for satisfactory low frequency response. However, the various leakage paths in such a microphone are apt to have impedances which are lower in value than the impedance of the amplifier input circuit. Consequently, the leakage paths may determine the cut-off frequency and defeat the advantages which might have been gained through use of the high impedance input circuit. In the past, this difficulty has been overcome to some extent by connecting in parallel a plurality of such series-connected piezoelectric units. Such construction has been found to introduce acoustical errors since it increases the size of the microphone. By means of the present invention it is possible to increase the effective resistance of the leakage paths to the point where it is no longer necessary to resort to the use of multiple sets connected in parallel. Accordingly, the invention makes it' possible to avoid many of the acoustical errors mentioned above, and to place the cutoff frequency at a lower frequency than could, in many cases, be obtained by placing a number of sets in parallel.

The invention is also of commercial importance in connection with low frequency devices such as pressure pickups of the type disclosed in Gravley Patent No. 2,207,539, since by avoiding the effects of stray leakage paths, the pickup may be made to respond to very slow fluctuations in pressure. In instruments of this type, leakage has heretofore been a serious limiting factor in obtaining low frequency response because .of the cutoff frequency which is imposed on the device by the leakage.

In view of the foregoing brief description of the invention it will be understood that it is one object of the invention to provide a method and apparatus for decreasing the undesired effects of stray leakage paths by increasing the effective resistance of all or certain of them.

It is another object to lower the cutoff frequency imposed on high impedance generators by all or certain of their stray leakage paths.

It is still another object to improve the performance of high impedance generators of the multiple type by raising at least the eifective low-frequency impedance of the leakage paths which impose a cutoff frequency upon the generator.

It is a further object of the invention to use the output of an amplifier to raise the impedance of the leakage path of a high impedance generator whose signal controls the amplifier.

These and other objects will be apparent from the following detailed description of the invention taken in connection with the accompanying drawings.

Fig. 1 illustrates the fundamental principles of the invention through a symbolic representation of a main generator and a separate auxiliary generator.

Fig. 2 illustrates an embodiment of the invention wherein the leakage path occurring between the terminals of a high impedance generator is treated in accordance with the invention by obtaining suitable guard potentials from a transformer connected in the output of an amplifier which is controlled by the signal of the high impedance generator.

Fig. 3 illustrates another practical embodiment of the invention similar to that of Fig. 2, suitable guard potentials being obtained from a potentiometer which forms part of the output circuit of the amplifier.

Fig. 4 is a detailed illustration of an embodiment of the invention wherein a two-stage feedback amplifier is connected to a pressure-type piezoelectric microphone, leakage between the terminals of the microphone being avoided by suitably controlling the potential of a guard electrode which forms a part of the microphone structure.

Fig. 5 illustrates a further embodiment wherein a remotely-located high impedance generator applies its signal through a coaxial cable to the control terminals of a single-stage inverse-feedback amplifier designed to prevent the direct current plate voltage from being applied to the terminals of the generator, suitable guard potentials for the guards of the cable, generator and amplifier being obtained from the feedback impedance.

Fig. 6 illustrates a further embodiment which uses an inverse feedback push-pull amplifier.

Fig. 7 illustrates an embodiment of the invention wherein the signal generator consists of a plurality of individual generators connected together in series, the output of the entire series of generators being applied to a single-stage inverse-feedback amplifier.

Fig. 8 is a perspective view which illustrates a commercial signal generator of the multiple type, and shows how the guard electrodes of the circuit of Fig. 7 may be arranged in the generator to reduce signal leakages therein.

Fig. 9 is an enlarged fragmentary cross-sectlon taken in the vicinity of section 99 of Fig. 8.

Fig. 10 is an enlarged fragmentary cross-section similar to Fig. 9, and shows a modified construction.

Fig. 11 is a partially schematic representation of a modification of the invention wherein a multiple generator and an associated amplifier are arranged to guard against aggravated leakage currents which would otherwise tend to flow to ground.

Fig. 12 shows the schematic wiring diagram of the assembly of Fig. 11.

Referring now to Fig. l, a high impedance generator I, having internal impedance Z, supplies signal potentials to terminals 2 and 3. A

conductive stray leakage path between terminals 2 and 3 is illustrated symbolically by the resistance 4. For purposes of explanation it will be assumed that any suitable load 5 is connected at its terminals 6 and 1 to the generator terminals 2 and 3. From what has been said above, it will be understood that since generator I has a high internal impedance Z, it is necessary that load 5 have an impedance which is maintained high relative to the impedance Z if best performance is to be obtained. It will further be understood that the leakage path 4 introduces adverse effects upon the performance of the generator unless its effective resistance is also maintained high relative to impedance Z. If it be assumed for purposes of explanation that the resistance of leakage path 4 is small relative to the impedance Z, then it will be understood that the problem presented by the circuit is to increase the effective resistance of that leakage path to the point where it is substantially ineffective in impairing the performance of the generator. For this purpose, the invention contemplates the insertion of a guard electrode 8 in the leakage path 4 so as to be in conductive relationship with the signal currents flowing in the leakage path and to divide the path into two portions which are on opposite sides of the guard. An auxiliary signal generator 9 is connected in shunt with a portion of the leakage path. For purposes of illustration, the auxiliary generator 9 is connected between guard 8 and terminal 3 so as to be in shunt with the intervening portion of the leakage path. I

In order that the objects of the invention may be accomplished most fully, the auxiliary gen:

erator 9 should be capable of maintaining guard 8 at substantially the instantaneous potential of terminal 2. When this condition is met combetween terminal I and guard l. The effectiveresistance of the leakage path will then be infinite. This follows from the fundamental relationship between resistance, voltage and current expressed by the following formula:

r; where R is the effective resistance of the leakage path which shunts the generator, E1 is the potential difference which exists at any instant between terminals 2 and 3, and where I1 is the current flowing between terminal 2 and guard I at the instant when El is measured. When the guard is maintained at the same alternating potential as terminal 2, there is no potential difference tending to cause a current to flow between them. The current Ii is accordingly zero, and R then equals and is infinite. If, however, the instantaneous potential of the guard is slightly less than the potential of terminal 2 at the same instant, a potential gradient exists between the latter and the guard with the result that some signal current flows through the leakage path. The effective resistance of the leakage path may however remain high, although being of a finite value, if the current Ii is sufficiently small. That is, if the potential difi'erence between terminal 2 and guard 8 is reduced appreciably below the value which it would have if generator 9 were disconnected from the guard, then the efi'ective resistance of the leakage path 4 will have been increased. It will be appreciated that the physical state of the leakage path (and hence its conductance) is not aifected by the invention even though its eifective resistance is increased in the manner just explained. From what has been said above, it will be understood that when the potential gradient has been suitably adjusted, the signal current flowing in the path may be reduced, and the effective resistance of the path, as seen by the generator, will have been increased. Accordingly,= it is of no significance that the path remains conductive, since the flow of current in the path may be controlled by the potential gradient which exists between the termini of the path.

It should be remembered that generator 9 supplies an alternating current signal; the potential of terminal 2 consequently fluctuates or alternates between positive values and negative values in accordance with the frequency characteristics of the signal. If the potential of guard 8 is to be held at exactly the potential of terminal 2, it is obvious that the potential of the guard must likewise fluctuate in exactly the same manner as the potential of the terminal. In other words, the potential of guard 8 must be continuously in exact phase relationship with the potential of terminal 2 so that their potentials are continuously of the same magnitude and instantaneous polarity. When this condition prevails, the two potentials will be in fully opposed phase relationship so far as current is concerned, since no signal or leakage current will flow between them. It will be understood that if the potentials are not in such exact phase relationship, guard 8 may reach the same maximum potential as terminal 2 but at a different instant. Under these conditions there will be a potential difference between the two points at any given instant and a Signal current will flow between them. The same result occurs if the potentials are in phase but of dlfierent magnitude. The fact, however, that a signal current flows between them does not signify that the invention is ineflective in accomplishing its objects since the current which flows when generator 9 is in operation may be less than the current which flows when the generator is disconnected from the guard. Any reduction in the leakage currents accomplished through .the use of potentials of opposed phase relationship represents an improvement over conditions which prevail without the invention.

It will be appreciated that in practice it is somewhat difiicult to employ a separate generator as the auxiliary source of guard voltage where the close phase relationship explained above must be maintained. Although it is possible to synchronize two generators, it is generally more expedient to use an amplifier as the auxiliary generator. That is, the high impedance generator may be connected to an amplifier in such manner that the generator signal controls the amplifier. The output signal of the amplifier may then be used in whole or in part to supply-the guard potentials. When an amplifier is used in this manner, the close phase relationships which are required in practicing the invention may be maintained quite readily. The following figures of the drawings illustrate various embodiments of the invention wherein amplifiers are so used.

Fig. 2 illustrates an embodiment in which the high impedance signal generator la, having terminals 2a and 3a, is connected to the input terminals l0 and ii of an amplifier l2. An amplified signal corresponding to the signal produced by the generator la may be obtained from the output terminals l3 and I4. If the internal construction of the amplifier i2 is unknown, it may nevertheless be used by connecting the primary ii of a suitable transformer l6across the terminals l3 and H. The signal available from the secondary ll of the transformer may be applied to the leakage path 4a by a pair of guard electrodes l8 and i9. In such a circuit, suitable polarities for the guard electrodes may be obtained by simply interchanging the connections between the guard electrodes and the secondary winding II. If the turns ratio of the transformer is suitably selected, then the potential of guard l8 may be maintained at substantially the instantaneous potential likewise the potential of guard I! may be maintained at substantially the potential of terminal in. When such a condition has been established in the leakage path, substantially no signal current will flow between terminal 2a and guard l8, nor between guard l9 and terminal 3a. In other words, the effective resistance of the leakage path, insofar as generator la is concerned, will have become infinite. It will, of course, be understood from what has been said above that the turns ratio of transformer l6 may be given values which do not establish an infinite resistance in the leakage path but which yet increase the effective resistance of the path in relation to the resistance which the path exhibits when the guard electrodes are not connected to the transformer. Those skilled in the art will recognize that the circuit arrangement of Fig. 2 may, under some conditions, be incapable of of terminal 2a andeliminating signal currents from the leakage path for the reason that the amplifier and/or the transformer may introduce some phase shift. This condition may, of course, be overcome by introducing suitable phase shifting means into the guard circuit or elsewhere so as to restore the properphase relationships between the guard electrodes and the generator terminals. Those skilled in the art will also realize that even though the output signal is in phase with the generator signal, certain frequency characteristics of the amplifier and/or the transformer may cause the magnitude of the output signal to depart, in greater or less degree over a limited range of frequencies, from the magnitude which it should have. That is, the ratio of output voltage to generator voltage may vary with frequency instead of having the constancy which the invention requires for optimum results. Such frequency variations in magnitude can be compensated by introducing suitable equalizing circuits. Since such circuits form no part of the invention, and are well known in the art and fully understood, they need not be discussed in further detail. Inthis connection, it should be understood that leakage is more serious at low frequencies than at high frequencies where capacitive generators are concerned, so that the corrections accomplished by such circuits or phase-shifting means may only be needed at the low frequencies.

When it is known that an amplifier has one terminal which is common to both its input and output circuits, and that its output is substantially in phase with the input, then the invention may be practiced by connecting the generator to the amplifier in accordance with the circuit shown in Fig. 3, due regard being given to the respective polarities. In this circuit, the high impedance generator lb, having terminals 2b and 3b, is connected to the input terminals lob, llb

of amplifier I211. The output of the amplifier may be obtained between terminals 13b and Nb. If, for purposes of explanation, it is assumed that terminals Nb and Nb are physically separate but are connected together within the amplifier, then the invention may be-practiced by connecting a suitable tapped resistance 20 across the output terminals, and a lead may be carried out from a suitable tap 2| thereon to a guard electrode 8b disposed in the leakage path between generator terminals 2b and 3b. It will be recognized that by means of this circuit, guard electrode 8b may be givenany potential intermediate the signal potentials of terminals I31) and 142) by tapping the resistance at an appropriate point. Accordingly, the potential difference between guard 8b and terminal 2b may be suitably decreased to effect the desired increase in the efiective resistance of the leakage path. As pointed out previously in connection with Fig. 2, the amplifier may introduce some phase shift so that suitable phase shifting means may also be needed in the circuit when it is desired to obtain nearly completely elimination of signal current from the leakage path. Likewise, equalizing circuits may be needed, as previously explained.

Fig. 4 illustrates the manner in which the present invention may be applied to a commercial high impedance generator. For purposes of illustration, a pressure-type piezoelectric microphone having the construction disclosed by. C. E. Semple, Jr., in U. S. Patent, No. 2,168,809, has been selected. Guard electrodes have, however,

lead wire 21'.

been inserted in the structure to overcome leakage between the lead wires which form the terminals of the piezoelectric unit. A two-stage amplifier, having a high impedance input circuit, is shown connected to the microphone together with the necessary circuit means required to overcome the leakage at the microphone terminals. The microphone consists of a square flexing-type piezoelectric unit 22 having a pair of yokes 23, 23 disposed on opposite sides of the piezoelectric unit and secured to diagonally opposite corners thereof. The midpoints of the yokes are secured to opposed conical diaphragms 24', 24' composed preferably of metal. The peripheries of the diaphragms are secured in suitable manner, as by cementing, to opposite faces of a ring-shaped framemember 25. Leads 26, 26 pass through a pair of openings in the wall of frame 25. The lead wires may be substantially sealed into the openings so as to prevent moisture from penetrating into the inner structure of the microphone.

If, when the microphone is first assembled, suitable precautions are taken (such as drying the air within the diaphragms and sealing the lead wires into the frame), leakage through paths which are inside the structure may be reduced to a negligible quantity. However, conductive leakage paths may exist on the outer surface of frame 25 between the leads 21, 21. In order to reduce such leakage, each of the leads is surrounded with a metal sleeve 28, 28' respectively, where the leads pass through the frame. Insulating bushings 28, 29' keep the sleeves out of electrical contact with the leads. The lead wires may be suitably sealed within the bushings for the purpose mentioned above. Lead wire 21 is connected to input terminal I00 and thence to the grid of the first stage 30 of the amplifier I20, and lead wire 21 is connected to input terminal He and thence to the cathode through a biasing resistance 3| which is shunted with a bypass condenser 32. The output of the first stage is applied between the grid and cathode of the second stage 33. 'A suitable blocking condenser 34 prevents the direct current potential of the plate circuit from being applied to the grid of the second stage. The second stage 33 is of conventional design with the exception that it supplies signal voltage to a potentiometer 200 which is connected across its output terminals [30 and H0. The potentiometer is tapped at an appropriate point He and a lead connects this point to guard terminal and thence to guard sleeve 28. The grid leak resistance 35 of the first stage is connected between the guard terminal 8c and input terminal Hlc. It will be seen that the signal potential drop which 'exists between the point He and output terminal He is applied between the guard sleeve 28 and Since the signal voltage across potentiometer 200 is substantially in phase with the generator signal voltage, a point can be found on the potentiometer which will maintain the potential of the guard sleeve 28 at substantially the instantaneous potential of lead wire 21 so that the effective resistance of the leakage path between them may be nearly infinite. It will be noted, however, that grid leak resistance 35 is connected between lead 21 and the guard terminal 80. Because of this, it might be thought that the effective resistance of the leakage path could not be made to exceed the resistance of the grid leak. It will be remembered, however, that under optimum conditions there is little or no aaeaaro 5 signal potential drop acrossthe ma ms, and little or no current fiowl through it. Accordingly, the presence of this element, even though it is a conductor, does not prevent the establishment of infinite resistance in the leakage path as seen by the generating element of the microphone. This feature forms the subject matter of the copending application of J. P. Arndt, Jr., referred to above, and a full disclosure of the feature will be found there.

It will be understood that if the microphone shown in Fig. 4 were to be connected to an amplifier in the manner shown in Fig. 2. then both guard sleeves ll and II would be required and they could be connected" to the secondary winding i'! of transformer I.

Fig. 5 illustrates another embodiment of the invention wherein any suitable high impedance alternating current generator supplies signal through a guarded coaxial cable to a remotelylocated single-stage inverse feedback amplifier. The coaxial cable illustrated here by schematic representation is of conventional construction except that it includes a flexible guard conductor 36 positioned in the cable intermediate the fiexible axial conductor 31 and the outer flexible conductor 38. These conductors are, of course,

suitably insulated from each other by flexible insulating materials, and the outer conductor 35 is covered with similar material, all as will be understood through reference to conventional coaxial cables. The amplifier lid illustrated here is of the type disclosed fully in the application of J. P. Arndt, Jr., mentioned above, and

represents an amplifier having a high input impedance. The signal generator id supplies alternating signals to its terminals 2d and id. Terminal id is connected to the axial conductor 31 of the cable, thence to input terminal iild and thence to the grid of the amplifier tube 3!. Termi'nal id is connected to the outer conductor 38 of the cable, thence to input terminal lid, and

thence to the cathode of the tube through a feedback resistance network ll, 40" and a biasing resistor 4i. It will be noted that the output of the amplifier is obtained between the terminals lid and lid and represents the signal current drop across the resistances l0, ll. This same signal potential drop is in circuit with the grid leak resistance 35d so that the resistance network and the grid leak resistance are in series, and are in shunt with the generator terminals ltd and lid. Accordingly, there is little signal potential difference between the ends of the grid leak resistance lid with the result that the effective input impedance of the amplifier is conversely high.

Fig. 5 shows. by way .of schematic representation that signal current leakage paths 4d, 42 and 43 exist between terminals 2d and 3d; between axial conductor 31 and outer conductor 38; and between input terminals illd and lid, respectively. In order to increase the efiective resistance of these respective leakage paths, a suitable guard electrode is positioned in each of them. These guard electrodes are designated lid, 35 and 44 respectively, and are connected to each other and to an appropriate terminal 45 of the amplifier. A connection is made between the latter terminal and the cathode end of the resistance network 40, III. In other words, the output voltage of the amplifier is applied between termini of a portion of each leakage path. As a result, the signal voltage which exists across the balance-of each leakage path represents the difi'erence between the generator voltageand the output voltage of the amplifier, and is much smaller than the voltage which would exist across it if the generator output were not so applied. Therefore the leakage current through the balanceof each path is proportionately reduced, and

the effective. impedance of each path is proportionately increased.

It will be recognized that the output signal obtained at terminals lld, lld must necessarily be zero (and thereby to obtain an infinite resist-' ance in the leakage path) with the amplifier which is illustrated in Fig. 5. Nevertheless, the design of the amplifier can be chosen so asto make the potential difi'erence between the grid and terminal lld quite small. For example, it is easily possible to have this potential difierence less than one tenth the voltage between terminals 2d and 3d. This signifies, of course, that if guard lid, for example, is positioned so as to divide leakage path Id into two equal parts, the effective resistance of the leakage path will have been increased more than five times.

It is a feature of the amplifier circuit of Fig. 5 that the direct current plate voltage is prevented from reaching the signal generator id. This is accomplished through the use ofthe'resistance network 40, 40' and the blocking condenser This feature is more fully discussed in the J. P. Arndt, Jr., application previously referred to. Inasmuch as piezoelectric generators should not be subjected to biasing direct current voltages, it will be recognized that this feature of the circuits makes them particularly useful in combination with piezoelectric generators. Inview thereof, it will be apparent that the microphone shown in Fig. 4 illustrates a commercial piezoelectric device which could be substituted for the schematic generator Id of Fig. 5.

Fig. 6 illustrates another embodiment of the invention wherein an amplifier having high input impedance supplies guard potentials to a pair of guard electrodes. The amplifier illustrated in this figure is similar in many respects to the amplifier of Fig. 5 in that it is of the inverse feedback type. The two amplifier tubes 41' are connected in push-pull relationship, however, so that a large proportion of the output of the amplifier which is available between terminals in and He, may be applied between a pair of guard electrodes its and i9e. For the same reason stated in connection with Fig. 5, the output signal voltage of this push-pull amplifier must necessarily be somewhat less than the signal voltage which is applied at the input of the amplifier. In consequence thereof, it is not possible to obtain an infinite effective resistance in the leakage path 4e since there will always be some slight potential difference between terminal 2e and guard l8e, and between terminal 3e and guard l9e. Nevertheless, the eflective resistance of the leakage path may be greatly increased through the use of this invention, as will be understood from the previous discussion. It will be apparent that since the push-pull amplifier of this figure does not apply biasing direct current voltages to its input terminals, it is well and 54.

suited for use with piezoelectric generators. It is especially useful in combination with the microphone of Fig. 4 since the guard potentials at terminals |8e and |9e may be connected to both of the guard sleeves 28, 25'of the microphone.

Fig. '7 illustrates an embodiment of the invention wherein the various leakage paths of a multiple generator are treated in accordance with the invention to reduce their adverse effects. In the figure, three signal generators 48, 49 and 50 are connected in series-to terminals 5|, 52, 53 The generators areof the high internal impedance type, and have respective impedance values represented by the symbols Z Z, and Z For purposes of explanation, it is considered that there is a conductive leakage path between each terminal and every other terminal of the group. These leakage paths are illustrated symbolically by the resistance paths 55, 55, 51, 55, 59 and 55. In order to accomplish the objects of the invention, guard electrodes 5|, 52, 53 and 54 are placed in the vicinity of terminals 5|, 52, 55 and 54 respectively, so as to lie in conductive relation to the various leakage paths which emanate from the terminal which the guard protects. The total signal of the generator is obtained between terminals 5| and 54 and is applied to aninverse feedback amplifier of the type described above in connection with Fig. 5. It consists of a vacuum tube 55 having a grid 55, a cathode 51, and a plate 58. A grid leak 59 is connected in series with a resistance network which consists of resistances l and 1| in parallel, to form a seriesparallel resistance circuit which is in shunt with terminals and 54 of the generator. The network resistances l5 and 1| constitute inverse feedback impedances which are common to the input and output circuits of the amplifier, one end of the network being connected to cathode 51 through a biasing resistance 12 and a bypass condenser 13, the other end of the network being connected through a battery 14 to the plate 58. A blocking condenser 15 keeps the. direct current of the plate circuit from being applied to the terminals of the generator. The output of the amplifier is available between terminals 15 and I1 and represents the signal current drop across the parallel network resistances I0 and I I. Guard 5| is connected by a lead to either the cathode end of resistance 15 at the point a, or to any selected point on resistance 12. Guard 52 is connected at point D to resistance 10, guard 53 is connected thereto at point c. Guard 54 is not connected and is unnecessary, but is shown since it would ordinarily be present in a commercial structure even though it is not used. This would ordinarily be the case because it is easier to apply the guard on a production line than to attempt to differentiate there between units which need all the guards and units which need fewer guards.

The operation of the circuit will be understood from the preceding discussion of Fig. 5, from i which it will be recognized that the output signal of the amplifier which is available at terminals Hi and 11 is slightly less than the signal applied to the amplifier from terminals 5| and 54 of the generator, the difference between these two signals being the signal which controls the grid of the vacuum tube. In view of this fact, it will be recognized that the signal potential drop across resistance 10 is close to but slightly less than the total signal voltage of the generator sothat it is possible to find points b and c which have potentials equal to the signal potentials of terminals 52 and 53 respectively. Likewise, guard 5| can be connected to resistor 15 or resistor I2 so as to be established at substantially the potentialof terminal 5 I Considering now the effect of guard 52, it will be noted that this guard electrode intercepts each of the three leakage paths 51, 58 and 55 which emanate from terminal 52. Since guard 52 is maintained at the same potential as terminal 52, it is clear that there is no potential gradient between the guard and the terminal to cause signal current to flow in the intervening portion of any one of the said leakage paths. In like manner, guard 53 intercepts each of the leakage paths emanating from terminal 53 and prevents flow of signal current therein. The effect of guard 5| is similar but since its potential is necessarily slightly less than the potential of terminal 5|, small leakage current will tend to flow in the paths 55, 55 and 51 between guard 5| and terminal 5|. Such currents are, however, materially less than if guard 5| were disconnected. It will accordingly be understood that generator leakage between the various terminals of the generator may be greatly decreased, the only leakage paths of any consequence being those which emanate from terminal 5| and extend to guard 5|. In any event, the leakage of generator signals may be so reduced as to render it substantially ineil'ective in imposing an undesired low frequency cutoff upon the generator. Of course, it will be apparent that the potentials of the guards may be adjusted so that there is a uniform potential difference between each guard and the terminal which it protects. It will also be apparent that while signal current flows. in the leakage paths between guards, this current is supplied by the amplifier and not by the signal generator.

From the foregoing discussion, it will be understood that the inverse feedback amplifiers illustrated in Figs. 5 and 6 are of such design that complete elimination of leakage is not possible. On the contrary, an amplifier of the type Shown in Fig. 4 affords complete elimination of leakage currents. It will therefore be appreciated that if a multiple generator of the type just discussed were connected to an amplifier of the latter type, then leakage in all of the leakage paths of the multiple generator could be avoided by tapping resistor 200 at appropriate places. In this connection, those skilled in the art will recognize that which the amplifier of Fig. 4 makes it possible to provide guard voltages which equal the generator voltages, its organization is such that it may become unstable under some conditions of adjust ment. On the other hand, the amplifiers of Figs. 5 and 6 always remain stable.

One mode of employing the invention in a practical construction will be understood more fully through reference to the perspective drawing of Fig. 8. In this figure, three sound cell microphone units of the type disclosed in Williams U. S. Patent No. 2,126,438 and Sawyer U. S. Patent No. 2,105,010 are secured together to form a single physical structure but are electrically connected together in series so that the total signal voltage of the assembly is the sum of the individual signals of the three units. In order to permit impinging sound waves to actuate each of the sound cells, the cells are spaced apart an appreciable distance but are secured together at their four corners. In order to minimize leakage between the terminals of the various units, suitable guard electrodes encircle each of the units so as to intercept as many as possible of the leakage paths. If, for purposes of explanation,

. it is considered that lead m corresponds to terminal II of Fig. 7, then metal band Ila corresponds to the guard terminal II. In like manner, lead 92a corresponds to terminal 52, and metal bands "a and 92b together correspond to guard 92. Similarly, bands 99a and 99b correspond to guard 99, and lead a corresponds to terminal 54. Fig. 8 illustrates a convenient way in which to secure the various generators together so as to simplify the electrical connections as much as possible. As described in the patents mentioned above, each sound cell unit may consist of a frame II, having an inner recess in which is disposed a pair of bimorph units 19 and 90 of the bending type. The construction of such bimorphs" is disclosed in Sawyer U. S. Reissue Patent No. 20,213. The piezoelectric units are spaced apart by means of soft material such as soft rubber pads 9|, ll disposed adjacent the leads Ila and Ila. The piezoelectric assembly as thus constructed is disposed within the recess of the frame 19. To secure the assembly therein and to shield the inner side of each unit from the effects of'implnging sound waves, sheets 92 and II of thin fiexible material such as paper are cemented to opposite faces of the frame 18 and to the outer faces of the piezoelectric units. In order to provide sufllcient freedom of movement, the sheets of paper are embossed so as to contain a rib 94 so positioned in the sheet as to lie between the margins of the piezoelectric units and the surrounding walls of the recess of the frame. As shown in the drawings, the leads Ia and 520 are brought out of the assembly on opposite sides thereof and between the frame and the paper sheets. In order to minimize leakage between the various terminals of the assembly, each terminal is encircled with a metal band as described above. It will be noted that the paper sheets serve to insulate the leads from the guard bands.

In orderto secure the three sound cells together and to provide a structure which has as few 'leakage paths .as possible, the cells are superimposed in spaced relationship and firmly clamped together by means of posts which join the cells together at their corners. The posts also make electrical connections between appropriate guard electrodesw The construction of the posts will be apparent through reference to Fig. 9, where it will be seen that screw 85 passes through the comer of the frame 18 and isolamped tightly thereto by means of nuts 99', 99. The nuts make electrical contact with guard 92a and therefore screw 85 must be insulated from screw 91 since guard 92:: should not be connected to guards 63a or "D. On the contrary, guard 93a should be connected electrically to guard 93b, so that screw 91 may be used to join cells 19' and 19" together at the corner here shown. In order to insulate screw 09 from screw 01, an insulating spacer 89 is threadably secured to the adjacent ends of screws 05 and 91 between frames 19 and I8.

The operation of the device and the function of the various guard electrodes will be understood from the discussion of Fig. 7.

Fig. illustrates a modified construction of the multiple generator illustrated in Fig. 8. In accordance with the construction of Fig. 10, both of the metallic guard bands of each generator are omitted. This can be done in some instances because the leakage between the pair of terminals of each individual generator may be negligible. It has been found, however, that even though this may be true of a particular construclugs 99 and 99 tion, undesirable leakage paths may exist between terminals of adjacent generators. Thus in Fig. -7, leakage between terminals II and 92 may be negligible while leakage between terminals II and 53 or between terminals 52 and It may be appreciable. Referring now to Fig. 8, it will be seen that if leakage occurs between the latter pairs of terminals, the leakage currents must necessarily travel along the comer posts in going iIOm generator 19 to generator II or from 19' to 19". In view thereof, it is apparent that the corner posts may be utilized as guard electrodes by suitably connecting them into the guard cirsuit. The construction shown in Fig. 10 accomplishes this purpose by providing for electrical connections to each of the screws in the various corner posts. It will be apparent that the construction of Fig. 10 is similar to that of Fig. 9 with the exception that the metal guard bands have been omitted and connecting lugs 09 and have been provided. By comparison with Fig- 8 it will be seen that screw ll takes the place of guard band 6 la insofar as the latter is capable of preventing leakage between adjacent generators. Likewise, screw 91' takes the place of guard hands "a and lib for the same purpose. Thus respectively would be connected to the associated amplifier in the same manner as guards GI and 93 are connected to the amplifier in Fig. 7. It will be understood that the other corner posts of the assembly may likewise be suitably connected to the associated amplifier so as to utilize the screws thereof as guards which correspond to guard 62 of Fig. 7.

The assembly shown in Fig. 11 illustrates a further modification of the invention and shows one mode of using the invention for the purpose of avoiding particular leakage currents. In this assembly, three piezoelectric microphones of the diaphragm type are connected together in series so that their total output is applied to an assoperiphery of shell 9|.

ciated amplifier. The microphones consist of a metal cup-shaped shell 9| in which is disposed a square torque type piezoelectric unit 92 secured at three of its corners to the bottom wall of shell 9| in accordance with the principles set forth in Williams U. S. Patent No. 2,105,011. One way of securing the piezoelectric unit to'the shell is by cementing three corners thereof to small discs 93 of damping material and cementing the discs in turn to the shell. The unsecured fourth corner 94 of the piezoelectric unit is attached to a conical diaphragm 95 which has its periphery cemented or otherwise suitably secured to the I The output of the piezoelectric unit 92 is made available at terminals 96 and 91. Each of these terminals consists of a metal rivet or screw which passes through shell 9| and is suitably insulated therefrom by means of an insulating bushing 99. Metal connecting lugs 99 and I00 are secured to the terminals 96 and 9'! respectively. The three microphones are suitably secured to aninsulating panel MI in such manner that their respective shells 9|, 9Ia and 9Ib are not in electrical contact with each other. Electrical connections are made between the various terminals of the generators so as to accumulate their respective outputs. The total output of the three generators is available bestood that the output voltage of the amplifier is necessarily slightly less than the input voltage which is applied to it at its input terminals since the output voltage plus the signal voltage drop across grid leak resistor l 01 equals the input voltage. The-output .voltage represents the signal voltage drop across the parallel resistors I08 and I08. A part of the signal voltage drop across these resistors is applied to guard circuits associated with the three microphones 9|, Ola and 9lb. Reference may be had to Fig. 12 for a more complete recognition of the features here involved.. For convenience in cross reference between the two figures, corresponding parts bear' the same reference characters. By reference to Fig. 12 it will be seen that each of the metal shells Ma. and Slb is used as a guard electrode and that the shells are each maintained at different signal potentials by connecting them to appropriate points H0, HI and H2 respectively. Each of these latter points is chosen so as to have its signal potential intermediate the signal potentials which exist between the terminals of the generator which is connected thereto. Thus the potential of point H2 is intermediate the potentials of terminals 96b and 91b. Correspondingly, the potential of point II I is intermediate the potentials of terminals 96a and 91a, and point I H! has a potential which is intermediate the potential of terminals 96 and 91.

In order to appreciate the advantages of the assembly illustrated in Figs. 11 and 12, it is necessary to compare the operation of the assembly with the operation which would be obtained if the three microphones were connected in series in conventional manner. Where several generators are employed and are connected in series for the purpose of accumulating their individual outputs, it is customary to ground the metal shell of each generator. If, for purposes of illustration, it be assumed that terminal I03 is grounded and that each of the microphones generates one millivolt, then it will be apparent that ingly, shell Sla is held at a potential of one and one-half millivolts since terminal 91a has a potentialof one millivolt and terminal 96a has a potential of two millivolts. Similarly, shell Si is held at a potential of two and one-half millivolts since terminal SIhas a potential of two millivolts and terminal 98 has a potential of three millivolts. It will be recognized that under such conditions a potential of one-half millivolt exists between each terminal and the adjacent shell with the result that the over-all leakage of the multiple generator is greatly reduced. For example, the leakage between terminal 96 and shell 9| is one-sixth of what it would be if shell 9| were grounded since the voltage tending to cause leakage currents to flow between terminal 96 and shell 9| is one-sixth of what it would be if the shell were grounded.

It will be recognized that no attempt is made in the assembly of Fig. 11 to avoid or reduce the leakage between the terminals of the individual generators. From the preceding discussion of the invention, however, it should be readily apparent that suitable guard electrodes could be inserted in the microphone structure and suitably connected to the associated amplifier to avoid such inter-terminal leakage. The description of Fig. 11 has not been encumbered with such additional guard electrodes since it has been found that in when shell 9lb is grounded, it will be at the same potential as terminal 91b while a potential difference of one millivolt will exist between it and terminal 96b. If shell Bla is grounded, a potential difference of two millivolts will exist between it and terminal 96a. In the same manner, if shell 9| is grounded, a potential difference of two millivolts ,will exist between it and terminal 91, and a potential difierence of three millivolts will exist between it and terminal 96. Recognizing now that leakage is apt to occur between the various terminals and the adjacent metal shell, it will be apparent that since the potential diiference between shell SI and terminal 96 is three millivolts, the leakage across insulator 98 is apt to be three times as much as the leakage between terminal 96b and shell 9!?) and is apt to be one and onehalf times as much as the leakage between terminal 96a and shell 91a. In other words, when the various shells are all grounded, the leakage across the nth generator of a series of similar generators is n times the leakage of the first generator of the series. While the leakage of the first unit may be negligible, a leakage of n times this value may be detrimental. When, however, the invention is applied as here shown, such aggravated leakages may be avoided. In accordance with this invention, shell 9 lb is maintained at a potential substantially midway between the potentials of terminals 96b and 911), or at a potential of one-half millivolt if the output of generator 91b is one millivolt. Correspondtion, as previously pointed out. In describing the operation of the assembly of Figs.'11 and 12, it was indicated that the points H0, Ill, H2 should be chosen to give the microphone shells a potential which is midway between the potentials of its terminals. Such adjustment is convenient and gives desirable results, but it should be recognized that the potential of the shell may be held at a higher value or a lower value, depending on the amount of correction that is desired.

This is equally true when other types of amplifiers are substituted for the amplifier illustrated in Figs. 11 and 12.

Since numerous changes may be made in the previous constructions in order to accommodate them to desired needs or uses, and since different applications of the invention may be made and since various omissions, substitutions and changes may be made by those skilled in the art without departing from the spirit of the invention, therefore, it should be recognized that all matter contained in the foregoing description or shown in the accompanying drawings is provided for the purpose of illustrating and explaining the invention and not to limit or restrict the invention. The scope of the invention should be ascertained from the following claims. The expression opposed phase relationship is used in the foregoing description and in the following claims to denote that condition in which the auxiliary or guard voltage opposes, and thereby reduces, the fiow of signal current from the primary signal generator through the leakage path to which the auxiliary or guard voltage has been applied.

I-Iaving now disclosed the invention, what I claim is:

- 1. In combination, a high impedance signal generator adapted to supply a signal potential andhaving a signal current leakage P th which conducts signal currents and thereby tends to reduce the signal available from said generator; and means for reducing the loss of signal resulting from said leakage, said means comprising an amplifier having. an input circuit and an output circuit, said amplifier having its input circuit connected to the signal output of said generator and having at least a part of the output signal of its output circuit appliedin opposed phase relationship to a part of said leakage path to reduce the flow of generator signal current through said leakage path.

2. The combination as claimed in claim 1 wherein said generator is an electrostatic generator having a low frequency cutoff as a result of the flow of leakage signal currents between its terminals.

3. The combination as claimed in claim 1 wherein the output applied to the leakage path is at least a part of the output-signal voltagedrop across a feedback impedance located in the said input circuit.

4. In combination, a high impedance signal generator adapted to supply a signal voltage at its terminals and having in shunt with said terminals a leakage path which conducts signal currents and thereby tends to reduce the signal available from said generator; and means for tials, said paths conducting signal currents and thereby tending to reduce the signal available from the main terminals of said generator; and means for reducing the loss of signal resulting from said leakage, said means comprising: at least one guard electrode disposed in each of said leakage paths in which leakage current is to be reduced; and an amplifier having an input circuit and an output circuit, said amplifier having its input circuit connected to said main generator terminals and having at least a part of the output voltage of its output circuit applied to each of said leakage paths, said output-signal being applied to each path between the guard electrode and an adjacent point on the same leakage path in opposed phase relationship to the voltage between the termini of said path, to maintain the voltage between the guard and point of each path greater than the voltage established between them by the fiow of leakage current in said leakage path,

reducing the loss of signal resulting from said leakage, said means comprising at least one guard electrode disposed in said leakage path intermediate its termini, and an amplifier having an input circuit and an output circuit, said amplifier having its input circuit connected to the generator terminals and having at least a part of the output signal of its output circuit applied between the guard electrode and an adjacent point on the leakage path in opposed phase relationship to maintain the voltage between said guard and point greater than the voltageestabllshad between them-by the flow of said leakage current in said leakage path.

5. The combination as claimed in claim 4 wherein said adjacent point on the leakage path is one of said generator terminals.

6. In combination, a high impedance signal generator adapted to supply a signal voltage at its terminals and having in shunt with said terminals a signal current leakage path which tends to reduce the signal available from said generator; and means for reducing the loss of signal resulting from said leakage, said means comprising a pair of spaced guard electrodes disposed in said leakage path; and an amplifier having an input circuit and an output circuit, said amplifier having its input circuit connected to the generator terminals and having at least a part of the output voltage of its output circuit applied between said guard electrodes to maintain each guard individually at substantially the instantaneous signal potential of the generator terminal to which it is nearest.

7. The combination as claimed in claim 6 wherein one guard coincides with a terminal of the generator.

8. In combination, an electrostatic generating device which provides a signal voltage between a pair of main terminals and which has a plurality of signal current leakage paths extending between various termini of difIerent signal poten- 9. In combination, a high impedance generator adapted in supply an alternating current signal at its terminals and having a signal current leakage path which conducts signal currents between a pair of termini and thereby tends to reduce the signal available from said generator; and an auxiliary source of signal connected in shunt with a fractional part of the leakage path, said auxiliary source being adapted to increase the signal-current potential-drop across said fractional part when connected in shunt therewith, and thereby to decrease the potential drops across the remaining parts of the leakage path.

10. The combination asclaimed in claim 5) wherein the auxiliary source is the output of an amplifier whose input is connected to the high 12. The combination as claimed in claim 9 wherein said fractional part of the leakage path extends from one terminus of the path to a guard electrode disposed in said leakage path intermediate said termini.

13. The combination as claimed in claim 9 wherein said fractional part of the leakage path extends between a pair of guard electrodes disposed in said leakage path in spaced relation to each other and to the termini of said path.

14. In combination, a high impedance generator adapted to supply an alternating current signal at its terminals and having a signal current leakage path which conducts signal currents between a pair of termini and thereby tends to reduce the signal available from said generator; and an auxiliary signal connected in shunt with a fractional part of the leakage path and in opposed phase relationship to the potential difference between its said termini,

15. The combination as claimed in claim 14 wherein said auxiliary signal is controlled by the high impedance generator.

16. The combination as claimed in claim 14 wherein the auxiliary signal is the output of an amplifier whose input is connected to the high impedance generator.

17. In combination, a high impedance alternating current generator adapted to supply an alternating current signal at a pair of terminals, and impaired in its performance by a signal current leakage path which conducts signal currents between termini of difierent potentials and which thereb tends toreduce the signal available at said generator terminals; a pair of guard electrodes disposedin said leakage path in spaced relation to each other, thereby being disposed to acquire difierent potentialsas a result of the flow of leakage current in said path; means for supplying an auxiliary alternating signal which corresponds in frequency characteristics to the' signal of said high impedance generator and has an amplitude greater than thev voltage established between said guard electrodes by said leakage current; and circuit means applying said auxiliary signal between said guard electrodes in opposed phase relationship to the voltage between said termini, thereby establishing a voltage. between the guards which is greater than the voltage established between them by said flow of leakage current.

18. The combination as claimed in claim 17 wherein one of said guard electrodes coincides with a terminus of the leakage path. r

19. A method for improving the performance of an electrostatic generating device adapted to supply a signal voltage at a pair of terminals, and having signal current leakage paths which con tive to ground.

duct signal currents between termini of difler- 4 ent signal potentials and thereby tend to reduce the signal available at said terminals, said method comprising the steps of: interposing a guard electrode in a selected leakage path intermediate the termini thereof; supplying an auxiliary alternating signal which corresponds in frequency characteristics to the signal oi. said high impedance generator and has an amplitude greater than the voltage established by said leakage'current between said guard and an adjacent point in the same path; and applying said auxiliary signal between said guard and adjacent point so as to be in opposed phase relationship to the voltage between the termini of the selected path thereby to reduce the now of signal current in a t 20. In combination, an inverse feedback amplifier having a feedback resistor common to the input and output circuits; a plurality of high impedance generator units connected in series 21. The invention set forth in claim 4, additionally characterized in that the connection between the generator terminals and the input circuit of the amplifier comprises a coaxial multiconductor cable, and further characterized in that the guard electrode is constituted by an intermediate conductor in said cable.

22. Incombination, a high impedance signal generator adapted to supply a signal voltage at its terminals, a leakage path connected in shunt relationship to said terminals, said leakage path being adapted to conduct signal currents and to tend to reduce the signal potential available from said generator; an amplifier having an input circuit and an output circuit; means including a multi-conductor coaxial cable connecting the terminals of the signal generator to the input circuit of the amplifier; a guard electrode constituted by an intermediate conductor in said coaxial cable, said guard electrode being disposed in said leakage path; said amplifier having at least a part 01 the output signal of its output circuit applied between the guard electrode and an adjacent poInt in the leakage path in Opposed phase relationship to maintain the voltage between said guard and point greater than the voltage established between them by the flow of leakage current in said leakage path, thereby reducing the loss of signal resulting from said leakage.

23. The invention set forth in claim 22, additionally characterized in that one terminal of the generator is connected to the innermost conductor in said coaxial cable, the other terminal of the generator is connected to the outermost conductor of said cable and the guard electrode is disposed intermediate said two conductors. I

24. In combination, a high impedance signal generator adapted to supply a signal voltage at its terminals; anamplifier having an input circuit and an output circuit; a coaxial cable comprising an inner conductor, an outer conductor and an intermediate guard conductor; connections between the terminals of the generator and the inner and outer conductors, respectively; connections between said inner and outer conductors, respectively, to the input terminals of the amplifier, and connections whereby at least a to accumulate their individual outputs, each of said generator units having a metal shell through which the generator leads pass; circ applying the accumulated output of the generators to the input of the amplifier; and circuit means connecting each of said shells to a diflerent pointon said common feedback resistor, the point to which each generator shell is connected having a signal voltage relative to uit means P rt of the output signa appearing in the output circuit is applied between'the guard conductor and the outer conductor in such phase relationship to potentials applied from the generator to said amplifier input circuit that the flow of current from the generator to the coaxial cable is minimized.

' WALTER J. BROWN. 

